1. Field of the Invention
The present invention relates to prism beam expanders commonly used for magnification in one dimension of laser beams, and more particularly to achromatic prism beam expanders with very high magnifications in lasers using diffraction gratings as narrow line width reflectors.
2. Description of Related Art
The prism beam expander (PBE) has been in wide spread use in laser related applications that require expansion or contraction of a laser beam in one dimension. Using a PBE, a round laser beam can be transformed into an elliptical laser beam. For instance, in a common application of the PBE, it is used to expand a laser beam in a dimension orthogonal to the lines in a grating prior to diffraction of the beam by the grating. This expansion prior to diffraction by a grating in tunable lasers significantly improves laser linewidth at a small cost in laser efficiency. Also, the prism beam expander is simple to align as compared to other methods for expanding laser beams.
Application of prism beam expanders in lasers is illustrated in U.S. Pat. No. 4,016,504 entitled OPTICAL BEAM EXPANDER FOR DYE LASER; Inventor: Klauminzer, issued Apr. 5, 1977, and U.S. Pat. No. 4,255,718 entitled TRANSVERSELY PUMPED DYE LASER HAVING IMPROVED CONVERSION EFFICIENCY; Inventor: Herbst, issued Mar. 10, 1981.
In the prior art, the magnification achieved by prism beam expanders has been limited to in the neighborhood of 50 times. This limited magnification has been a constraining factor on the laser linewidth in tunable lasers using diffraction gratings.
Because of this limit in magnification in prior art prism beam expanders, the manufacturers of tunable lasers that require very narrow linewidths have been creating complicated laser cavity designs, such as the "grazing incidence grating dye laser". These complicated cavity designs lead to alignment difficulties in manufacture that are undesirable.
Another feature of prism beam expanders that is desirable for many laser related applications is achromaticity. The design of achromatic prism beam expanders has been addressed in detail in the prior art. See Trebino, "The Achromatic N-Prism Beam Expander: Optical Configurations" dated October 1984, submitted to Applied Optics. The Trebino paper describes the design of achromatic prism beam expanders with maximized transmission characteristics. According to Trebino, a high magnification prism beam expander that is achromatic should be configured with all apex angles oriented on a first side (labelled up U) of an optical path with the last apex angle oriented on the opposite side (labelled down D) of the optical path (UUU...UD). Trebino concludes that this design should be applied for all prism beam expanders with magnifications greater than (2-1/(2.sup.N-1 -1)).sup.N, where N is equal to the number of prisms in the PBE. Thus a 6 prism expander with magnification greater than 60 should use the UUUUUD configuration according to Trebino. The recommended configuration of Trebino suffers the disadvantage that the input beam and the output beam are misaligned substantially. This complicates manufacture of optical devices relying on the prism beam expander due to the irregular beam angles resulting from passing through this PBE configuration.
It is desirable, as illustrated in the Herbst reference cited above, to have an input beam to the PBE nearly collinear with the output beam of the PBE.
In summary, according to the prior art, achromatic prism beam expanders are useful for applications requiring magnification much less than 100 times, and to manufacture higher magnification PBEs, an awkward configuration and large numbers of prisms would be required. It is desirable, however, to achieve very high magnifications in achromatic PBEs with nearly collinear input and output beams for many applications.